Rolling magnesium alloy



ROLLING MAGNESIUM ALLOY Edmund C. Burke, Midland, Mich., and Carl M. Zvanut, Alton, TIL, assignors to The Dow Chemical Company, Midland, Mich, a corporation of Delaware No Drawing. Application September 28, 1956 Serial No. 612,887

3 Claims. (Cl. 148-11.5)

The invention relates to an improved method of pro-v ducing rolled sheet of magnesium-base alloy. It more particularly concerns an improved method of producing rolled sheet in coiled form of a strain hardenable magnesium-base alloy.

Heretofore magnesium-base alloys of the strain hardenable type have been rolled into sheet to a limited extent from moderate size cast ingot, the sheet produced being short enough to be handled as individual pieces without coiling.

Insofar as We are aware, there is no commercially available method by which magnesium-base alloys of the strain hardenab-le type can be rolled into a long sheet requiring a coiling operation which does not exhibit severe localized twinning or its concomitant discontinuous grain growth upon subsequently uncoiling to make it flat and heating it as required in proceeding with the usual bending and forming operations. The localized twinning or discontinuous grain growth adversely affects the smoothness of the sheet as well as its mechanical properties. Accordingly, it is the principal object of the invention to provide an improved method of producing rolled sheet of the foregoing alloys which does not exhibit these adverse properties. Other objects and advantages will appear as the description of the invention proceeds.

The invention is predicated upon the discovery that by producing most the reduction in thickness of the alloy by rolling at a temperature above the recrystal lization temperature, preferably cooling the so-rolled metal to a temperature somewhat above its minimum recrystallization temperature, then finishing to gauge in a single pass While reducing the temperature of the metal to below that at which it recrystallizes followed by coiling, the so-coiled metal can be uncoiled so as to make it flat and submitted to all the usual forming operations requiring heating the metal, or a limited amount of additional rolling, without producing localized twinning or discontinuous grain growth.

In carrying out the invention, cast ingot of a strain hardenable magnesium-base alloy is employed. Examples of these alloys and their ASTM designations are: AZ31 (nominal composition: 3 percent Al, 1 percent Zn, 0.2 percent Mn, balance Mg); AZ61 (nominal composition: 6 percent Al, 1 percent Zn, 0.15 percent Mn, balance Mg); HK31 (nominal composition: 3 percent Th, 0.7 percent Zr, balance Mg); HKll (nominal composition: 1 percent Th, 0.7 percent Zr, balance Mg); HM21 (nominal composition: 2 percent Th, 1 percent Mn, balance Mg); ZE41 (nominal composition: 4 percent Zn, 0.5 percent Zr, 0.4 R.E. (R.E. stands for metal of the rare earths), balance Mg); ZElO (nominal composition: 1 percent Zr, 0.12 percent R.E., balance Mg). The ingot is subjected to a solution heat treatment as by soaking in a furnace at a temperature at which substantially complete solution of the solid solution forming alloying metal is obtained. A soaking temperature of 850 F. generally suflices. Soaking time varies it States Patent evidence of twinning or sporadic grain growth.

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2 with the volume of the ingot and its shape and generally ranges from about 10 to 20 hours including the time taken to heat the metal from room temperature to the solution heat treatment temperature. hold the ingot at the solution heat treatment temperature for at least 4 to 6 hours.

rolling a magnesium-base alloy. The number of hot passes will vary according to the initial thickness of the ingot and the ultimate thickness desired. As the hot rolling proceeds, the rolled metal lengthens. This necessitates coiling the rolled metal if before the final pass sufficient space is not available to handle the sheet. In the event that coiling becomes necessary before the final pass the hot rolled metal is coiled while hot, uncoiled hot, and thereafter further rolled hot. If desired, the coiled hot rolled metal may be set aside and allowed to cool before further rolling. Before again rolling the so-cooled coiled metal, the coil is heated to a temperature above its recrystallization temperature. 7

In accordance with the invention, the hot rolling, that is the rolling which is done at a temperature at which the metal is above its recrystallization temperature, is discontinued when the thickness to which the metal is thereby reduced is about 118 to 143 percent or preferably about percent of the desired final thickness. The so-obtained metal is then rolled to its ultimate thickness in a single pass in a cold mill which provides conditions under which recrystallization does not occur. These require that the hot rolled metal be at a tempera-' ture of about 25 to 200 Fahrenheit degrees above its minimum recrystallization temperature when about to enter the mill and that the mill rolls be cooled so that the rolled metal will emerge from the mill at a suitably low temperature.

The cooling of the rolls may be accomplished by flowing a cooled liquid lubricant over the rolls or through passages in the rolls, so that the metal, as it is reduced in thickness to final gauge, loses. heat to the rolls and. emerges from the rolls at a temperature below that at which it recrystallizes, e.g. for AZ31 about 275 F.

As it leaves the mill, the metal, so-rolled to the final gauge, is coiled as by winding it upon a mandrel which is suitably revolved, the temperature of the metal as it is coiled being below its recrystallization temperature. The minimum radius of the coil so-obtained should not be less than about 50 times the thickness of the metal so.

coiled.

The so-coiled rolled metal is in the hard rolled condition and exhibits hightensile and compressive strength and other desirable properties. On being uncoiled to make it fiat and heated, as needed in forming operations, the metal is readily formable and does not exhibit undesirable twinning or discontinuous grain growth.- As a consequence, articles formed or drawn from the metal: taken from the coil exhibit a smooth surface free from The following example is illustrative of the practice of the invention:

A scalped cast ingot 12 inches thick, 40 inches wide, and 76 inches long weighing 2250 pounds of a magnesium-base alloy having the nominal composition of 3 percent Al, 1 percent zinc, 0.2 percent Mn, the bal- Pat ented Apr. 26, 1960.

It is desirable to mice being Mg was solution heat treated at 850 to 900 F. for about 16 hours. The so-treated ingot, thereby heated to about 850 F., was then subjected to hot rolling on a breakdown mill with, 27, inch diameter work rolls. 8 In the first passes, the ingot was passed between the rolls sideways which widened the ingot to 51 inches and at the same time reduced its thickness to 9.4 inches. A roll lubricant of a 10 percent water solution of a polyglycol was applied to the rolls the temperature of which were estimated to be about 140 F The roll surface speed was about 800 feet per minute. The so-rolled ingot was then turned 90 in the plane of the axes of the rolls and sent between the rolls in the endways direction in 18 passes which reduced the thickness to 0.18 inch. During these passes, the temperature of the rolled metal fell to about 550 F. and on leaving the mill at the 18th pass, the metal while still above 550 F. was led to the coiler having a mandrel 24 inches in diameter and thereon coiled. During the foregoing passes the metal widened about 3 to 4 inches and increased in length to about 320 feet. The coiled metal while still hot was placed in an oven at 650 F. to await further rolling in more passes to final gauge on a finishing mill. The first four of these passes were hot rolling passes at temperatures between 540 and 650 F., the metal being uncoiled as it entered the rolls and coiled again (mandrel diameter 24 inches) while hot, except on the 5th pass, as it emerged from the rolls. The first of these passes reduced the thickness from 0.18 inch to 0.14 inch, the second from 0.14 to 0.108, the third from 0.108 to 0.088, and the fourth from 0.088 to 0.070 inch. The so-hot rolled metal on emerging from the finishing rolls on the fourth pass was coiled and then cooled to between 380 and 450 F. After so-cooling the coiled metal, it was then given the fifth and final or finishing pass on the finishing mill which reduced the thickness from 0.070 to 0.052 inch while the temperature of the metal was thereby reduced to between 220 and 260 F. This temperature lowering was accomplished by transferring heat from the metal to the rolls by cooling the rolls to a temperature between about 100 and 120 F. and by adusting the speed of the rolls to about 500 feet per minute surface speed so that the metal was retained between the rolls long enough to bring about the desired temperature lowering. As the so-cooled and rolled metal emerged from the mill, it was coiled using a 24 inch diameter mandrel while at a temperature between about 220 and 260 F. The uncoiled length of the so-produced sheet was about 1000 feet.

Samples of the so-rolled metal taken from the coil exhibited the following properties:

Strength in 1,000 p.s.i. Percent Condition E in 2 inches UTS TYS 1 GYS 1 as rolled. 42, 000 33, 000 28, 000 3 to 5 annealed 2 41, 000 31, 000 26, 000 15 article of a strain hardenable magnesium-base alloy in the recrystallized state to a preselected final thickness which comprises rolling between the rolls of a mill the article at a temperature above its recrystallization 5 temperature, so as to reduce its thickness to between about 118 and 143 percentof the desiredfinal thickness, rolling the so-reduced metal tothe desired final thickness while lowering the temperature of the metal being rolled by contact with the rolls of the mill to a temperature below that at which it recrystallizes, and coiling the resulting rolled. metal without reheating above its re crystallization temperature, the minimum radius of the coil so-produced being not less than about 50 times the thickness of the rolled metal.

2. The method of reducing the thickness of a rolled article of a strain hardenable magnesium-base alloy in the recrystallized state to a preselected final thickness which comprises rolling between the rolls of a mill the article at a temperature above its recrystallization temperature, the said temperature being not less than 400 F;, so as to reduce its thickness to between about 118 and 143 percent of the desired final thickness, cooling the so-obtained hot rolled metal to a temperature 25 to 200 Fahrenheit degrees above its minimum recrystallization temperature, rolling the so-cooled recrystallized metal to the desired final thickness while changing the temperature of the metal being rolled by contact with the rolls of the mill to a temperature below that at which it recrystallizes, and coiling the resulting rolled metal at a temperature below that at which it recrystallizes, the minimum radius of the coil so-produced being 'not less than about 50 times the thickness of the rolled metal.

3. The method of producing a coil of a rolled product of a strain hardenable magnesium-base alloy whereby upon uncoiling the coiled metal and heating the same for submission to a forming operation the metal does not exhibit localized twinning and discontinuous grain growth which comprises rolling between the rolls of a mill a. cast ingot of the magnesium-base alloy at a temperature above its recrystallization temperature, the said temperature being not less than 400 F. so as to reduce the thickness to within not less than about 118 to r 143 percent of the desired final thickness whereby the metal is subjected to conditions bringing about recrystallization during rolling, cooling the so-obtained hot rolled metal to a temperature 25 to 200 Fahrenheit degrees above its recrystallization temperature, rolling the socooled recrystallized metal to the desired final thickness while lowering the temperature of the metal being rolled by contact with the rolls of the mill to a temperature below that at which it recrystallizes, and coiling the resulting rolled metal at a temperature below that at which it recrystallizes, the minimum radius of the coil so-obtained being not less than about 50- tirnes the thickness of the coiled metal.

References Cited in the file of this patent v UNITED STATES PATENTS 2,029,728 Lowry et al. Feb. 4, 1936 2,294,648 Ansel et al. Sept. 1, 1942 2,314,010 McDonald Mar. 16, 1943 OTHER REFERENCES Iron Age, page 45, Mar. 18, 1943.

Introduction to Mg and Its Alloys, John Alico (Zitf- Davis Pub. Co.), pages 82-,-86, 1945;

Technology of Magnesium andIts Alloys, pages 100- 104, F. A. Hughes &1Co., 1940. 

1. THE METHOD OF REDUCING THE THICKNESS OF A ROLLED ARTICLE OF A STRAIN HARDENABLE MAGNESIUM-BASE ALLOY IN THE RECRYSTALLIZED STATE TO A PRESELECTED FINAL THICKNESS WHICH COMPRISES ROLLING BETWEEN THE ROLLS OF A MILL THE ARTICLE AT A TEMPERATURE ABOVE ITS RECRYSTALLIZATION TEMPERATURE, SO AS TO REDUCE ITS THICKNESS TO BETWEEN ABOUT 118 AND 143 PERCENT OF THE DESIRED FINAL THICKNESS, ROLLING THE SO-REDUCE METAL TO THE DESIRED FINAL THICKNESS WHILE LOWERING THE TEMPERTURE OF THE METAL BEING ROLLED BY CONTACT WITH THE ROLLS OF THE MILL TO A TEMPERATURE BELOW THAT WHICH IT RECRYSTALLIZES, AND COILING THE RESULTING ROLLED METAL WITHOUT REHEATING ABOVE ITS RECRYSTALLIZATION TEMPERATURE, THE MINIMUM RADIUS OF THE COIL SO-PRODUCED BEING NOT LESS THAN ABOUT 50 TIMES THE THICKNESS OF THE ROLLED METAL. 